1. Field of the Invention
This invention relates to a resistance-variation type moisture sensor comprising a moisture sensitive film of a polyelectrolyte, which detects variation of humidity in the atmosphere by means of a variation in the resistance of the moisture sensitive film based on the water-molecular absorption or the water-molecule desorption thereof.
2. Description of the Prior Art
As a moisture sensitive material wherein electrical resistance or electrical capacitance varies depending upon a variation in humidity or water vapor in the atmosphere, there have been, for example, a moisture sensitive material having a sintered body of metal oxides such as iron oxide (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4), tin oxide (SnO.sub.2), etc., or a metal oxide film; a moisture sensitive material having a hydrophilic polymer film or a polyelectrolyte; a moisture sensitive material having an electrolyte salt such as lithium chloride (LiCl); and a moisture sensitive material having a hygroscopic polymer film in which conductive particles or fibers such as carbon are dispersed.
While moisture sensors containing a metal oxide film or a polyelectrolyte film generally have a wide moisture-sensitivity range, their resistance varies exponentially, responding to relative humidity in the atmosphere. Moisture sensors containing a metal oxide have an excellent heat resistance and respond rapidly, but they have a high temperature resistance coefficient. Especially, moisture sensors having a sintered body of metal oxides are inferior in reproducibility and/or interchangeability of the moisture sensitive characteristic thereof because the moisture sensitive characteristic depends upon the constituents of the sensor to a great extent. Moisture sensors having an electrolyte salt such as lithium chloride detect only humidity in a narrow range and if they are allowed to stand in a highly humid atmosphere for a long period of time, the electrolyte salt therein is eluted or diluted resulting in deterioration of the moisture sensitive characteristic of the sensor, and accordingly it cannot be used for the determination of high humidity. Moisture sensors having a hygroscopic resin or the like, in which conductive particles or fibers are dispersed, cannot detect a humidity in a wide range because they exhibit a steep variation of the resistance thereof in a highly humid atmosphere, while it is not sensitive to low humidity. Also a moisture sensor having a hydrophilic polymer film or a polyelectrolyte film is inferior in moisture resistance, water resistance and durability, so that if it were allowed to stand in a highly humid atmosphere or in dew for a long period, its moisture sensitive characteristic would be changed, while it is advantageous in that it operates in a wide moisture sensitive range, has a rapid moisture sensitive response, a simple structure, and is easily produced at a low cost. A moisture sensor having a hydrophilic polymer film or a polyelectrolyte film is also inferior in heat-resistance, so that if it were allowed to stand at a high temperature (e.g., 100.degree. C.), its moisture sensitive characteristic would deteriorate and/or change.